Add state setter

Author: PIG208

lib/model/message.dart

@@ -48,13 +48,16 @@ class OutboxMessage {
 
   OutboxMessageLifecycle get state => _state;
   OutboxMessageLifecycle _state;
-  void markAsSent() {
-    assert(_state == OutboxMessageLifecycle.sending);
-    _state = OutboxMessageLifecycle.sent;
-  }
-  void markAsFailed() {
-    assert(_state == OutboxMessageLifecycle.sending);
-    _state = OutboxMessageLifecycle.failed;
+  set state(OutboxMessageLifecycle value) {
+    // See [OutboxMessageLifecycle] for valid state transitions.
+    switch (value) {
+      case OutboxMessageLifecycle.sending:
+        assert(false);
+      case OutboxMessageLifecycle.sent:
+      case OutboxMessageLifecycle.failed:
+        assert(_state == OutboxMessageLifecycle.sending);
+    }
+    _state = value;
   }
 }